In an internal combustion engine, this type of piston has a crown, a skirt, a pair of gudgeon pin bosses, and a pair of inserts made of a material of lower thermal expansion than that of the light metal, such as steel. Each insert is and shaped so that at least part of the insert conforms to the inside shape of the piston skirt, whereby to form a bi-metallic element with the light metal of the piston in order to control the expansion of the piston skirt, when the piston is heated during engine operation, each insert has a pair of bands extending around the inside of the piston skirt from the region of the gudgeon pin boss, and the bands of said pair are joined by a connecting portion of the insert in the region of the gudgeon pin boss.
In a prior method of making such a piston, permanent magnets are firmly fixed in recesses of the mold core where the inserts are to be set, and the inserts are positioned firmly on the mold core during casting operation by the magnetic force. However, the magnetic force decreases gradually due to high temperature during casting operation and then the inserts tend to fall off from the mold core when a molten metal is poured into the mold cavity.
When the piston is cast by the squeeze casting process, in which a very high pressure around 1,000 kg/cm.sup.2 is applied in the molten metal, the heat flux from the molten metal into the mold core is accelerated because of better contact between the molten metal and the mold core. Therefore, the magnet is exposed to higher temperature and the magnetic force decreases very rapidly. Consequently, the magnet on the mold core for squeeze casting must be replaced at every 500 to 1000 casting cycles, which increases the production cost of the squeeze-cast piston with cast-in steel inserts remarkably.
In a second prior method of making such a piston, two setting pins per one insert are fixed in the mold core. Two setting holes are drilled at the corresponding position of the insert, and the inserts are positioned on the mold core by the engagement of setting pins on the mold core and holes of the inserts.
In gravity die casting process, the mold core for the piston is normally divided into several pieces, so that the mold core can be freely taken out from the piston casting after the molten metal solidifies, even if the piston inside has under-cut portions of if there is such engagement of setting pins and holes of the inserts.
In case of squeeze casting process, the mold core must be of one piece, because a very high pressure on the moltenmetal would press the melt into every gap between the divided mold core pieces, which would hinder the mold core from taking out from the piston casting. Thus, it is impossible to apply such setting pins for casting pistons with steel inserts by squeeze casting process.